Electrohydrodynamic-Jet-Printed SnO2-TiO2-Composite-Based Microelectromechanical Systems Sensor with Enhanced Ethanol Detection

Ethanol sensors have found extensive applications across various industries, including the chemical, environmental, transportation, and healthcare sectors. With increasing demands for enhanced performance and reduced energy consumption, there is a growing need for developing new ethanol sensors. Micro-electromechanical system (MEMS) devices offer promising prospects in gas sensor applications due to their compact size, low power requirements, and seamless integration capabilities. In this study, SnO2-TiO2 nanocomposites with varying molar ratios of SnO2 and TiO2 were synthesized via ball milling and then printed on MEMS chips for ethanol sensing using electrohydrodynamic (EHD) printing. The study indicates that the two metal oxides dispersed evenly, resulting in a well-formed gas-sensitive film. The SnO2-TiO2 composite exhibits the best performance at a molar ratio of 1:1, with a response value of 25.6 to 50 ppm ethanol at 288 °C. This value is 7.2 times and 1.8 times higher than that of single SnO2 and TiO2 gas sensors, respectively. The enhanced gas sensitivity can be attributed to the increased surface reactive oxygen species and optimized material resistance resulting from the chemical and electronic effects of the composite.


Introduction
As an important organic compound, ethanol not only plays a key role as a solvent and organic synthesis reactant in traditional industries, but also has a wide range of applications in antifreeze, fuel, medical disinfection and other fields [1][2][3].However, ethanol is flammable and can easily cause fires when exposed to high temperatures or flames.In addition, ethanol is easy to volatilize to form vapor and mix with air to form an explosive mixture [4][5][6].Therefore, effective detection of ethanol vapor is particularly necessary.The resistance-based gas sensor is one of the research hotspots for fast ethanol detection.Among different sensing materials, metal oxide semiconductors (MOSs) can change the resistance in different kinds and concentrations of gases and have many advantages, such as high sensitivity, fast response, low cost, etc.However, traditional MOS-based sensors using electric heating wire suffer from high energy consumption and low response and consistency, necessitating the development of new materials and devices [7][8][9][10][11].
Micro-electro-mechanical systems (MEMS) devices have been attracting increasing attention in MOS gas sensors due to their low energy consumption, device miniaturization, and reduced device variation [12][13][14][15].Most micro hot plate chips used contain substrate, microheater, and sensing electrodes.The sensitive MOS materials are then deposited on the surface of the micro hot plate for gas sensing.To date, there are three commonly used deposition methods for MEMS gas sensor gas-sensitive materials: dispensing, printing, and SnO 2 (50-70 nm, 99.99%) was purchased from Shanghai Macklin Biochemical Technology Co., Ltd.(Shanghai, China); TiO 2 (rutile type, 25 nm, 99.8%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.(Shanghai, China); ethylene glycol (AR), anhydrous ethanol (AR), anhydrous methanol (AR), and formaldehyde (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China); CO, NH 3 , and H 2 were purchased from Shanghai Weichuang Standard Gas Analysis Technology Co., Ltd.(Shanghai, China).All chemicals were analytical grade chemicals from Sinopharm Chemical Reagent Company and were used without further purification.
The typical process of preparation of SnO 2 -TiO 2 slurry is shown in Figure 1a.Firstly, 0.65 g of SnO 2 powder and 0.35 g of TiO 2 powder are dispersed in 8.1 mL of ethylene glycol containing 0.1 g of dispersant.The obtained suspension then is ultrasonically dispersed for 30 min and transferred to a ball mill jar with a volume of 50 mL and a total of 56 grinding balls.The ball mill (QM-3SP04, Nanjing Nanda Instrument Co., Ltd., Nanjing, China) parameters are set to a frequency of 25 Hz and a milling time of 720 min.The milled slurry is then collected for later use.In this sample, the molar ratio of SnO 2 to TiO 2 is 1:1, and this sample is named ST1-1.Similarly, by adjusting the molar ratio of SnO 2 to TiO 2 to 3:1 and 1:3, the samples are denoted as ST3-1 and ST1-3, respectively.In addition, for comparison, 1 g of individual SnO 2 and TiO 2 were ball-milled under the same conditions, denoted as SnO 2 and TiO 2 , respectively.The MEMS devices were purchased from Jipai Xiangxin (Shanghai) Intelligent Technology Co., Ltd.(Shanghai, China).The structure of the MEMS micro-hotplate containing interdigital electrode, isolation, heater electrode and supporting film is shown in Figure S1.The corresponding SEM image shows that the size of the square suspended membrane (ABCD) is about 150 × 150 μm.The gas-sensitive membrane is deposited on the surface of the interdigital electrode.The preparation process of the gas-sensitive membrane is shown in Figure 1b: Firstly, the ball-milled slurry is drop-coated onto the MEMS chip functional area using EHD technology (HEDJET-H, Wuhan Huawei Technology Co., Ltd., Wuhan, China).The MEMS chip with sensing materials then is dried at room temperature and annealed at 350 °C for 2 h with the heating rate of 2 °C/min.Subsequently, the obtained MEMS sensor ages at 1.8 V for 36 h to improve its stability.

Characterization of Materials
The microstructure of MEMS and gas-sensitive membranes was studied using field emission scanning electron microscopy (SEM, JSM-6700F).Transmission electron microscopy and high-resolution transmission electron microscopy (TEM and HRTEM, JEM-2100 Field Emission TEM) were used to investigate the microscopic morphology and crystal structure of the composite nanomaterials.The elemental distribution of the materials was studied using the energy dispersive X-ray spectroscopy (EDS) of the JEM-F200 Field Emission Transmission Electron Microscope.X-ray diffraction (XRD, Rigaku D/MAX2200) was used to analyze the crystal structure of the product, with a scanning angle ranging from 20° to 80° at a speed of 8°/min.X-ray photoelectron spectroscopy (XPS, Thermofisher Nexsa) was employed to measure the chemical state of surface elements, using aluminum as the target material, operating in a high vacuum environment of 5.0 × 10 −7 mbar, with a working voltage set at 12 kV and a filament current stabilized at 6 mA.To ensure the accuracy of the test results, the binding energy was calibrated using the C1s (284.80 eV) spectral line.The Brunauer-Emmett-Teller (BET)specific surface area of the samples was determined using an automatic specific surface area and porosity analyzer (Micromeritics ASAP 2460, USA).The pore size distribution was calculated from the adsorption branch of the nitrogen adsorption-desorption isotherm using the Barrett-Joyner-Halenda (BJH) method.Photoluminescence The MEMS devices were purchased from Jipai Xiangxin (Shanghai) Intelligent Technology Co., Ltd.(Shanghai, China).The structure of the MEMS micro-hotplate containing interdigital electrode, isolation, heater electrode and supporting film is shown in Figure S1.The corresponding SEM image shows that the size of the square suspended membrane (ABCD) is about 150 × 150 µm.The gas-sensitive membrane is deposited on the surface of the interdigital electrode.The preparation process of the gas-sensitive membrane is shown in Figure 1b: Firstly, the ball-milled slurry is drop-coated onto the MEMS chip functional area using EHD technology (HEDJET-H, Wuhan Huawei Technology Co., Ltd., Wuhan, China).The MEMS chip with sensing materials then is dried at room temperature and annealed at 350 • C for 2 h with the heating rate of 2 • C/min.Subsequently, the obtained MEMS sensor ages at 1.8 V for 36 h to improve its stability.

Characterization of Materials
The microstructure of MEMS and gas-sensitive membranes was studied using field emission scanning electron microscopy (SEM, JSM-6700F).Transmission electron microscopy and high-resolution transmission electron microscopy (TEM and HRTEM, JEM-2100 Field Emission TEM) were used to investigate the microscopic morphology and crystal structure of the composite nanomaterials.The elemental distribution of the materials was studied using the energy dispersive X-ray spectroscopy (EDS) of the JEM-F200 Field Emission Transmission Electron Microscope.X-ray diffraction (XRD, Rigaku D/MAX2200) was used to analyze the crystal structure of the product, with a scanning angle ranging from 20 • to 80 • at a speed of 8 • /min.X-ray photoelectron spectroscopy (XPS, Thermofisher Nexsa) was employed to measure the chemical state of surface elements, using aluminum as the target material, operating in a high vacuum environment of 5.0 × 10 −7 mbar, with a working voltage set at 12 kV and a filament current stabilized at 6 mA.To ensure the accuracy of the test results, the binding energy was calibrated using the C1s (284.80 eV) spectral line.The Brunauer-Emmett-Teller (BET)specific surface area of the samples was determined using an automatic specific surface area and porosity analyzer (Micromeritics ASAP 2460, USA).The pore size distribution Sensors 2024, 24, 4866 4 of 15 was calculated from the adsorption branch of the nitrogen adsorption-desorption isotherm using the Barrett-Joyner-Halenda (BJH) method.Photoluminescence (PL) measurements were performed using a fluorescence spectrometer (Edinburgh FLS1000, UK) with an excitation source of a 290 nm Xe lamp.
Oxygen temperature-programmed desorption (O 2 -TPD) tests were conducted using a temperature-programmed chemical adsorption analyzer (Hiden DECRA, UK).The test conditions were as follows: the sample was pretreated by drying at a programmed temperature ramp from room temperature to 288 • C at 10 • C/min, purged with a He gas flow (30-50 mL/min) for 1 h, cooled to 50 • C, saturated with a 10% O 2 /He mixture (30-50 mL/min) for 1 h, purged with a He gas flow (30-50 mL/min) for 1 h to remove weakly physically adsorbed O 2 from the surface, and finally desorbed in a He atmosphere at a heating rate of 10 • C /min up to 290 • C. The desorbed gas was detected using a thermal conductivity detector (TCD).

Gas Sensitive Performance Test
The gas sensitivity test was performed on the electronic gas sensitivity tester smart gas sensitivity analysis system (LP-002A).One of the advantages of MEMS sensors is their low power consumption, and the relationship between power consumption and heating voltage is shown in Figure S2.For example, when the heating voltage is 2.1 V, the sensor power consumption is only 42 mW.The operating temperature of the MEMS sensor is controlled by varying the heating voltage of the heater, and the relationship between temperature and heating voltage is shown in Figure S3.During the experiment, the relative humidity of the laboratory was approximately 50%.The required amount of target gas is injected into the test chamber using a micro-injector.In the circuit for measuring gas response, the load resistance (R L ) is connected in series with the MEMS gas sensor (Figure S4).The circuit voltage (V) is set to 5 V, and the output voltage (V out ) is recorded 10 times per second.The relationship between MEMS sensor resistance R and V out is given by Equation (1).Therefore, R can be calculated from V out .
Sensor response is calculated using the following formula: where R a and R g represent the resistance of the sensor exposed to ambient air and target gas, respectively.The response time (T res ) and recovery time (T rec ) refer to the time required for the response change to reach 90% of the equilibrium value after the injection and removal of the tested gas, respectively.

Characterization Results of Gas Sensitive Materials and Devices
Figure 2a illustrates the suspended structure of the MEMS micro-hotplate, which is supported by four suspended beams in the sensing area.The area of the Pt interdigitated electrodes is 150 × 150 µm (Figure 2b).The power consumption of the MEMS sensor in working condition is approximately 42 mW, which is much lower than that of commercially available ceramic-based sensors.As observed from Figure 2c,d, the sensor prepared using EHD technology (ST1-1) forms a dense gas-sensitive film without visible cracks, completely covering the electrodes without spillage.Since gas sensitivity largely depends on the size and morphology of the sample, a uniform nanostructure can reduce measurement errors during the gas sensing process, thereby improving sensing capability and reproducibility.High-magnification SEM images of ST1-1 reveal a highly porous microstructure (Figure 2e) where nanoparticles are tightly packed together, forming a breathable microporous membrane (Figure 2f). Figure S5 shows SEM images of SnO 2 and TiO 2 sensors.The gas-sensitive films of both sensors also cover the functional areas of the chips well, further indicating that EHD can better control the distribution of gas-sensitive materials on the surface.However, slight cracks are visible in these two gas-sensitive films.Upon comparing them to the structure of ST1-1, it appears that the composite material aids in the formation of a uniform and dense gas-sensitive film.
Sensors 2024, 24, x FOR PEER REVIEW 5 of 15 Upon comparing them to the structure of ST1-1, it appears that the composite material aids in the formation of a uniform and dense gas-sensitive film.Figure 3a,b present TEM images of SnO2 and TiO2, respectively.It can be observed that the particle sizes of SnO2 and TiO2 after ball milling are around tens of nanometers, but the particles are not uniform and tend to agglomerate.Figure 3c demonstrates that the particle size of the ball-milled ST1-1 composite material is mainly concentrated at about 30 nm, indicating a more uniform distribution.ST1-3 and ST3-1 also show similar results (Figure S6). Figure 3d displays an HRTEM image of the ST1-1 composite material, showing a lattice spacing of 0.265 nm for SnO2 (101) and 0.169 nm for TiO2 (211), indicating a very close spatial contact between SnO2 and TiO2.EDS results confirm that the ST1-1 composite material consists of Ti, Sn, and O, as shown in Figure 3. Figure 3a-d   Figure 3a,b present TEM images of SnO 2 and TiO 2 , respectively.It can be observed that the particle sizes of SnO 2 and TiO 2 after ball milling are around tens of nanometers, but the particles are not uniform and tend to agglomerate.Figure 3c demonstrates that the particle size of the ball-milled ST1-1 composite material is mainly concentrated at about 30 nm, indicating a more uniform distribution.ST1-3 and ST3-1 also show similar results (Figure S6). Figure 3d displays an HRTEM image of the ST1-1 composite material, showing a lattice spacing of 0.265 nm for SnO 2 (101) and 0.169 nm for TiO 2 (211), indicating a very close spatial contact between SnO 2 and TiO 2 .EDS results confirm that the ST1-1 composite material consists of Ti, Sn, and O, as shown in Figure 3. Figure 3a-d  very close spatial contact between SnO2 and TiO2.EDS results confirm that the ST1-1 composite material consists of Ti, Sn, and O, as shown in Figure 3. Figure 3a-d    The surface properties of materials play a crucial role in determining gas sensing performance.Firstly, N2 adsorption-desorption isotherm tests were conducted to investigate the specific surface areas of SnO2, TiO2, and the ST1-1 composite (refer to Figure S8).As indicated in Table S1, the specific surface areas of SnO2, ST1-1, and TiO2 materials are 9.7 m 2 /g, 16.1 m 2 /g, and 25.0 m 2 /g, respectively.The specific surface area of the composite material is approximately the average of the individual oxides, suggesting that the composite material tends to neutralize the specific surface areas of the individual oxides.
X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface elemental composition and valence state distribution of SnO2, TiO2, and ST1-1.In the highresolution XPS spectrum of Sn 3d (Figure S9a), two prominent peaks can be clearly observed.Both the peaks of ST1-1 material and SnO2 material are centered at 486.9 eV (Sn 3d5/2) and 495.4 eV (Sn 3d3/2) without any shift, corresponding to Sn 4+ , indicating the presence of Sn in the +4 oxidation state [28,30].In the high-resolution XPS spectrum of Ti 2p (Figure S9b), two distinct peaks are visible.The peaks of both ST1-1 and TiO2 materials are centered at 458.4 eV (Ti 2p3/2) and 464.4 eV (Ti 2p1/2), corresponding to Ti 4+ , demonstrating the existence of Ti in the +4 oxidation state.However, it is evident that the peaks of the ST1-1 sample are slightly shifted towards lower binding energies compared to pure 3+ The surface properties of materials play a crucial role in determining gas sensing performance.Firstly, N 2 adsorption-desorption isotherm tests were conducted to investigate the specific surface areas of SnO 2 , TiO 2 , and the ST1-1 composite (refer to Figure S8).As indicated in Table S1, the specific surface areas of SnO 2 , ST1-1, and TiO 2 materials are 9.7 m 2 /g, 16.1 m 2 /g, and 25.0 m 2 /g, respectively.The specific surface area of the composite material is approximately the average of the individual oxides, suggesting that the composite material tends to neutralize the specific surface areas of the individual oxides.
X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface elemental composition and valence state distribution of SnO 2 , TiO 2 , and ST1-1.In the high-resolution XPS spectrum of Sn 3d (Figure S9a), two prominent peaks can be clearly observed.Both the peaks of ST1-1 material and SnO 2 material are centered at 486.9 eV (Sn 3d5/2) and 495.4 eV (Sn 3d3/2) without any shift, corresponding to Sn 4+ , indicating the presence of Sn in the +4 oxidation state [28,30].In the high-resolution XPS spectrum of Ti 2p (Figure S9b), two distinct peaks are visible.The peaks of both ST1-1 and TiO 2 materials are centered at 458.4 eV (Ti 2p3/2) and 464.4 eV (Ti 2p1/2), corresponding to Ti 4+ , demonstrating the existence of Ti in the +4 oxidation state.However, it is evident that the peaks of the ST1-1 sample are slightly shifted towards lower binding energies compared to pure TiO 2 , suggesting the formation of a portion of Ti 3+ .
The XPS spectrum of O 1s can be decomposed into two peaks, as shown in Figure 5a and Figure S10a.Generally, the peak located at 530.58 eV can be attributed to lattice oxygen (O L ), while the peak at 532.59 eV can be attributed to oxygen defects (O V ) on the surface.Table 1 summarizes the oxygen species of different samples.The percentage of oxygen vacancies (O V ) in ST1-1 is 54.8%, significantly higher than 31.9% for SnO 2 , 26.6% for TiO 2 , 33.6 for ST3-1, and 30.5% for ST1-3, indicating a higher concentration of oxygen vacancies in the ST1-1 sample, which can provide more sites for oxygen molecule adsorption.
Sensors 2024, 24, x FOR PEER REVIEW 7 of 15 The XPS spectrum of O 1s can be decomposed into two peaks, as shown in Figures 5a and S10a.Generally, the peak located at 530.58 eV can be attributed to lattice oxygen (OL), while the peak at 532.59 eV can be attributed to oxygen defects (OV) on the surface.Table 1 summarizes the oxygen species of different samples.The percentage of oxygen vacancies (OV) in ST1-1 is 54.8%, significantly higher than 31.9% for SnO2, 26.6% for TiO2, 33.6 for ST3-1, and 30.5% for ST1-3, indicating a higher concentration of oxygen vacancies in the ST1-1 sample, which can provide more sites for oxygen molecule adsorption.The optical absorptions of pristine SnO2, TiO2, and ST1-1 were tested using UV-Visible spectroscopy.The bandgap of a direct bandgap semiconductor can be calculated through the relationship between the Kubelka-Munk function (αhv) 2 and photon energy (hv), as shown in Figure 5b.The direct bandgap of SnO2 is 3.59 eV, and the direct bandgap of TiO2 is 2.86 eV.The direct bandgap of the composite ST1-1 material is 2.74 eV.Obviously, compared to pristine SnO2 and TiO2, ST1-1 has a narrower bandgap, which is possibly due to the formation of defective oxygen vacancies between the valence and conduction bands.Therefore, ST1-1 contains more oxygen vacancies and exhibits a wider range of light absorption.
Further investigation into the surface active oxygen was conducted using O2-TPD.According to literature reports, desorption peaks below 500 °C are typically attributed to surface-adsorbed active oxygen species, including physically adsorbed oxygen (OP) and  The optical absorptions of pristine SnO 2 , TiO 2 , and ST1-1 were tested using UV-Visible spectroscopy.The bandgap of a direct bandgap semiconductor can be calculated through the relationship between the Kubelka-Munk function (αhv) 2 and photon energy (hv), as shown in Figure 5b.The direct bandgap of SnO 2 is 3.59 eV, and the direct bandgap of TiO 2 is 2.86 eV.The direct bandgap of the composite ST1-1 material is 2.74 eV.Obviously, compared to pristine SnO 2 and TiO 2 , ST1-1 has a narrower bandgap, which is possibly due to the formation of defective oxygen vacancies between the valence and conduction bands.Therefore, ST1-1 contains more oxygen vacancies and exhibits a wider range of light absorption.
Further investigation into the surface active oxygen was conducted using O 2 -TPD.According to literature reports, desorption peaks below 500 • C are typically attributed to surface-adsorbed active oxygen species, including physically adsorbed oxygen (O P ) and chemically adsorbed oxygen (O C ), while desorption peaks above 500 • C correspond to the desorption of lattice oxygen.The results of the O 2 -TPD analysis are shown in Figure S7a.The SnO 2 material exhibits a desorption peak at 150 • C, but the peak intensity is relatively weak.The TiO 2 material shows a pronounced desorption peak at 230 • C, with a much stronger peak intensity compared to SnO 2 .Figure S10b indicates that ST1-3 with more TiO 2 has stronger desorption peak compared to that of ST3-1, which is due to TiO 2 having stronger oxygen adsorption capacity than SnO 2 , as shown in Figure 6a.The composite ST1-1 material displays a significant desorption peak at 200 • C, and its peak intensity is noticeably stronger than that of TiO 2 and SnO 2 , consistent with the XPS results.This suggests that SnO 2 and TiO 2 tend to form more surface defects during the ball milling process.These findings indicate that the ST1-1 material has more adsorbed oxygen on its surface.
Sensors 2024, 24, x FOR PEER REVIEW 8 of 15 chemically adsorbed oxygen (OC), while desorption peaks above 500 °C correspond to the desorption of lattice oxygen.The results of the O2-TPD analysis are shown in Figure S7a.The SnO2 material exhibits a desorption peak at 150 °C, but the peak intensity is relatively weak.The TiO2 material shows a pronounced desorption peak at 230 °C, with a much stronger peak intensity compared to SnO2. Figure S10b indicates that ST1-3 with more TiO2 has stronger desorption peak compared to that of ST3-1, which is due to TiO2 having stronger oxygen adsorption capacity than SnO2, as shown in Figure 6a.The composite ST1-1 material displays a significant desorption peak at 200 °C, and its peak intensity is noticeably stronger than that of TiO2 and SnO2, consistent with the XPS results.This suggests that SnO2 and TiO2 tend to form more surface defects during the ball milling process.
These findings indicate that the ST1-1 material has more adsorbed oxygen on its surface.Photoluminescence (PL) spectroscopy was further used to analyze the charge interaction between SnO2 and TiO2.Numerous studies have shown that PL peaks originate from the recombination of excited photogenerated electrons and photogenerated holes.Therefore, a lower PL peak intensity indicates a lower recombination rate of photogenerated electrons and holes.The results are shown in Figure 6.The PL peak intensities of SnO2 and TiO2 alone are relatively strong, but the PL peak intensity significantly decreases when they are combined, indicating the lowest peak intensity for the composite ST1-1.As for the SnO2-TiO2 sample (Figure S10c), the sample of ST1-3 with higher amount of TiO2 own stronger PL peak intensity, which is possibly due to TiO2 having a stronger PL peak intensity, as shown in Figure 6b.Above results suggest that electron transfer may occur after the combination of SnO2 and TiO2, thereby reducing the recombination rate of photogenerated electron-hole pairs.

Gas Sensitive Property
In the process of gas sensitivity performance testing, operating temperature has a significant impact on the reactions occurring on the surface and is an important parameter for gas-sensitive materials.Therefore, the effect of operating temperature on gas sensitivity performance was first studied.The response values of different sensors to 50 ppm C2H5OH were tested within a temperature range of 230 °C to 330 °C.The test environment was at room temperature with 50% relative humidity.The test results are shown in Figure 7a.As the operating temperature increases, except for the TiO2 sensor, the response of other sensors shows a volcanic trend that first increases and then decreases.For SnO2, its response values are relatively low throughout the entire tested temperature range.At the optimal operating temperature of 288 °C, the response value is 3.6.In contrast, the TiO2 sensor has a response value of 13.8 at the same temperature, indicating a higher response to ethanol.However, after combining the two materials, the response values have Photoluminescence (PL) spectroscopy was further used to analyze the charge interaction between SnO 2 and TiO 2 .Numerous studies have shown that PL peaks originate from the recombination of excited photogenerated electrons and photogenerated holes.Therefore, a lower PL peak intensity indicates a lower recombination rate of photogenerated electrons and holes.The results are shown in Figure 6.The PL peak intensities of SnO 2 and TiO 2 alone are relatively strong, but the PL peak intensity significantly decreases when they are combined, indicating the lowest peak intensity for the composite ST1-1.As for the SnO 2 -TiO 2 sample (Figure S10c), the sample of ST1-3 with higher amount of TiO 2 own stronger PL peak intensity, which is possibly due to TiO 2 having a stronger PL peak intensity, as shown in Figure 6b.Above results suggest that electron transfer may occur after the combination of SnO 2 and TiO 2 , thereby reducing the recombination rate of photogenerated electron-hole pairs.

Gas Sensitive Property
In the process of gas sensitivity performance testing, operating temperature has a significant impact on the reactions occurring on the surface and is an important parameter for gas-sensitive materials.Therefore, the effect of operating temperature on gas sensitivity performance was first studied.The response values of different sensors to 50 ppm C 2 H 5 OH were tested within a temperature range of 230 • C to 330 • C. The test environment was at room temperature with 50% relative humidity.The test results are shown in Figure 7a.As the operating temperature increases, except for the TiO 2 sensor, the response of other sensors shows a volcanic trend that first increases and then decreases.For SnO 2 , its response values are relatively low throughout the entire tested temperature range.At the optimal operating temperature of 288 • C, the response value is 3.6.In contrast, the TiO 2 sensor has a response value of 13.8 at the same temperature, indicating a higher response to ethanol.However, after combining the two materials, the response values have significantly improved compared to SnO 2 .Specifically, the response values of the ST3-1 and ST1-3 sensors at their optimal operating temperatures are 6.9 and 5.6, respectively, which are higher than that of SnO 2 but smaller than that of the TiO 2 sensor at the corresponding temperature.Nonetheless, when the molar ratio of SnO 2 to TiO 2 is 1:1, the ST1-1 sensor achieves a remarkable response value of 25.6 at the optimal operating temperature of 288 • C.This represents 7.2-fold and 1.8-fold increases compared to the response values of the individual SnO 2 and TiO 2 sensors, respectively.This significant enhancement indicates that combining SnO 2 and TiO 2 in an appropriate ratio can greatly improve the sensor's response performance.
sponding temperature.Nonetheless, when the molar ratio of SnO2 to TiO2 is 1:1, the ST1-1 sensor achieves a remarkable response value of 25.6 at the optimal operating temperature of 288 °C.This represents 7.2-fold and 1.8-fold increases compared to the response values of the individual SnO2 and TiO2 sensors, respectively.This significant enhancement indicates that combining SnO2 and TiO2 in an appropriate ratio can greatly improve the sensor's response performance.
Changes in gas-sensitive resistance were also observed when varying the operating temperature, and the results are shown in Figure 7b.At 233 °C, the TiO2 sensor exhibits the highest resistance, approximately 429 MΩ, while the SnO2 sensor has the lowest resistance of approximately 63 KΩ.When the two materials are combined, the resistance decreases as the amount of SnO2 added increases.As the temperature rises, the Ra of all samples decreases.Among them, the TiO2 sensor shows a significant decrease in resistance with increasing temperature, while the resistance of SnO2 remains notably lower throughout the entire tested temperature range.At the optimal operating temperature of 288 °C, the resistances of the Ra(SnO2), Ra(ST3-1), Ra(ST1-1), Ra(ST1-3), and Ra(TiO2) sensors are 25 KΩ, 33 KΩ, 516 KΩ, 8060 KΩ, and 19,800 KΩ, respectively.As the TiO2 content increases, the resistance gradually increases.Additionally, it's worth noting that high resistance can pose challenges for device circuit design.Therefore, combining SnO2 and TiO2 is beneficial for obtaining more practical resistance values.Selectivity is a crucial performance indicator for gas sensors in practical applications, as it determines the sensor's ability to recognize target gases and resist interference.Figure 8 shows the response values of different sensors under the influence of 50 ppm ethanol and common reducing interference gases such as CH3OH, HCHO, CO, NH3, and H2 at a working temperature of 288 °C.As can be seen from the figure, under the same conditions, the responses of SnO2 sensor to CH3OH, HCHO, CO, NH3, and H2 are 2.3, 1.4, 1.3, 1.7, and 3.9, respectively, while the response to C2H5OH is 2.9.This indicates that the SnO2 sensor does not have good selectivity for ethanol.The responses of TiO2 sensor to CH3OH, HCHO, CO, NH3, and H2 are 4.5, 2.1, 3.7, 3.2, and 4.1, respectively, while the response to C2H5OH is 13.8.This suggests that the TiO2 sensor has better selectivity for ethanol.When the two materials are combined, the selectivity of ST1-3 and ST3-1 does not significantly improve.The responses to different gases are low.However, the ST1-1 sensor shows a significant enhancement in responsiveness to ethanol.Its responses to CH3OH, HCHO, Changes in gas-sensitive resistance were also observed when varying the operating temperature, and the results are shown in Figure 7b.At 233 • C, the TiO 2 sensor exhibits the highest resistance, approximately 429 MΩ, while the SnO 2 sensor has the lowest resistance of approximately 63 KΩ.When the two materials are combined, the resistance decreases as the amount of SnO 2 added increases.As the temperature rises, the R a of all samples decreases.Among them, the TiO 2 sensor shows a significant decrease in resistance with increasing temperature, while the resistance of SnO 2 remains notably lower throughout the entire tested temperature range.At the optimal operating temperature of 288 • C, the resistances of the R a (SnO 2 ), R a (ST3-1), R a (ST1-1), R a (ST1-3), and R a (TiO 2 ) sensors are 25 KΩ, 33 KΩ, 516 KΩ, 8060 KΩ, and 19,800 KΩ, respectively.As the TiO 2 content increases, the resistance gradually increases.Additionally, it's worth noting that high resistance can pose challenges for device circuit design.Therefore, combining SnO 2 and TiO 2 is beneficial for obtaining more practical resistance values.
Selectivity is a crucial performance indicator for gas sensors in practical applications, as it determines the sensor's ability to recognize target gases and resist interference.Figure 8 shows the response values of different sensors under the influence of 50 ppm ethanol and common reducing interference gases such as CH 3 OH, HCHO, CO, NH 3 , and H 2 at a working temperature of 288 • C. As can be seen from the figure, under the same conditions, the responses of SnO 2 sensor to CH 3 OH, HCHO, CO, NH 3 , and H 2 are 2.3, 1.4, 1.3, 1.7, and 3.9, respectively, while the response to C 2 H 5 OH is 2.9.This indicates that the SnO 2 sensor does not have good selectivity for ethanol.The responses of TiO 2 sensor to CH 3 OH, HCHO, CO, NH 3 , and H 2 are 4.5, 2.1, 3.7, 3.2, and 4.1, respectively, while the response to C 2 H 5 OH is 13.8.This suggests that the TiO 2 sensor has better selectivity for ethanol.When the two materials are combined, the selectivity of ST1-3 and ST3-1 does not significantly improve.The responses to different gases are low.However, the ST1-1 sensor shows a significant enhancement in responsiveness to ethanol.Its responses to CH 3 OH, HCHO, CO, NH 3 , and H 2 are 8.3, 1.9, 2.4, 1.6, and 6.2, respectively, while the response to C 2 H 5 OH is 25.6, which is significantly higher than that for other gases.These results indicate that the ST1-1 sensor combines the advantages of both metal oxides, improving selectivity for ethanol vapor while lowering the optimal operating temperature.
Sensors 2024, 24, x FOR PEER REVIEW 10 of 15 CO, NH3, and H2 are 8.3, 1.9, 2.4, 1.6, and 6.2, respectively, while the response to C2H5OH is 25.6, which is significantly higher than that for other gases.These results indicate that the ST1-1 sensor combines the advantages of both metal oxides, improving selectivity for ethanol vapor while lowering the optimal operating temperature.The sensing performance of the ST1-1 sensor to ethanol at a working temperature of 288 °C was further investigated.Firstly, the response and recovery of the ST1-1 sensor to different concentrations (10-60 ppm) of ethanol were studied.The results, shown in Figure 9a, indicate that the response value increases linearly with increasing gas concentration from 10 ppm to 60 ppm, with a fitted line slope of 0.34 (Figure 9b).The response speed and recovery speed of the MEMS sensor can be evaluated by response time and recovery time.As shown in Figure 9c, the response time of the ST1-1 sensor at 288 °C is approximately 11.8 s, and the recovery time is approximately 60.1 s, indicating relatively fast response and recovery speeds of the ST1-1.The stability of the ST1-1 sensor is demonstrated in Figure 9d.There is no significant difference in the response values during six cycles of testing under 50 ppm ethanol conditions, indicating excellent signal reproducibility and short-term repeatability of the ST1-1 sensor.Figure 9e shows the long-term stability of the ST1-1 sensor tested over 30 days under the same environmental conditions with 50 ppm ethanol.No significant drift or downward trend was observed within 30 days, indicating good long-term stability.Additionally, the effects of humidity on sensing performance and initial resistance were studied.The changes in the response and initial resistance of the ST1-1 sensor are shown in Figure 9f, with an ambient temperature of 25 °C and a relative humidity (RH) range of 20-90%.Both parameters show small fluctuations in the low RH range of 25-55%, a significant decrease in the 55-75% range, and a slow decrease in the 75-90% range, requiring temperature drift correction.Consistency is also one of the important indicators of sensor performance.Figure S11 shows that three MEMS gas sensors prepared under the same conditions respond almost identically to 50 ppm C2H5OH, illustrating that EHD technology provides a reliable and effective method for preparing gas sensors with excellent consistency.9a, indicate that the response value increases linearly with increasing gas concentration from 10 ppm to 60 ppm, with a fitted line slope of 0.34 (Figure 9b).The response speed and recovery speed of the MEMS sensor can be evaluated by response time and recovery time.As shown in Figure 9c, the response time of the ST1-1 sensor at 288 • C is approximately 11.8 s, and the recovery time is approximately 60.1 s, indicating relatively fast response and recovery speeds of the ST1-1.The stability of the ST1-1 sensor is demonstrated in Figure 9d.There is no significant difference in the response values during six cycles of testing under 50 ppm ethanol conditions, indicating excellent signal reproducibility and short-term repeatability of the ST1-1 sensor.Figure 9e shows the long-term stability of the ST1-1 sensor tested over 30 days under the same environmental conditions with 50 ppm ethanol.No significant drift or downward trend was observed within 30 days, indicating good long-term stability.Additionally, the effects of humidity on sensing performance and initial resistance were studied.The changes in the response and initial resistance of the ST1-1 sensor are shown in Figure 9f, with an ambient temperature of 25 • C and a relative humidity (RH) range of 20-90%.Both parameters show small fluctuations in the low RH range of 25-55%, a significant decrease in the 55-75% range, and a slow decrease in the 75-90% range, requiring temperature drift correction.Consistency is also one of the important indicators of sensor performance.Figure S11 shows that three MEMS gas sensors prepared under the same conditions respond almost identically to 50 ppm C 2 H 5 OH, illustrating that EHD technology provides a reliable and effective method for preparing gas sensors with excellent consistency.We also compared the comprehensive sensing performance of our ST1-1 sensor with MEMS devices reported in the literature.As shown in Table 2, the ST1-1 sensor exhibits a higher response and lower power consumption compared to most reported materials.

Discussion of Gas-Sensitive Mechanism
The sensing mechanism of MOS gas sensors is related to the resistance changes caused by gas adsorption and desorption.The gas-sensing performance is closely associated with surface adsorbed oxygen, and variations in the amount of adsorbed oxygen can lead to changes the resistance of materials, enabling gas detection [39,40].
In this study, SnO2 and TiO2 were combined using a ball milling process.During ball milling, differences in the material properties of SnO2 and TiO2 can lead to the creation of surface defects.XPS results indicate that ST1-1 has more oxygen defects than SnO2 and TiO2 (the oxygen vacancy contents of SnO2, TiO2, and ST1-1 are 31.9%,26.6%, and 54.8%,We also compared the comprehensive sensing performance of our ST1-1 sensor with MEMS devices reported in the literature.As shown in Table 2, the ST1-1 sensor exhibits a higher response and lower power consumption compared to most reported materials.

Discussion of Gas-Sensitive Mechanism
The sensing mechanism of MOS gas sensors is related to the resistance changes caused by gas adsorption and desorption.The gas-sensing performance is closely associated with surface adsorbed oxygen, and variations in the amount of adsorbed oxygen can lead to changes the resistance of materials, enabling gas detection [39,40].
In this study, SnO 2 and TiO 2 were combined using a ball milling process.During ball milling, differences in the material properties of SnO 2 and TiO 2 can lead to the creation of surface defects.XPS results indicate that ST1-1 has more oxygen defects than SnO 2 and TiO 2 (the oxygen vacancy contents of SnO 2 , TiO 2 , and ST1-1 are 31.9%,26.6%, and 54.8%, respectively), which is also supported by UV-Visible absorption spectroscopy.Many studies have shown that surface oxygen defects can significantly promote the formation of surface-active oxygen.The O 2 -TPD test results further demonstrate that the amount of adsorbed oxygen on the composite gas-sensing material increases significantly, playing a crucial role in enhancing the gas-sensing performance of ST1-1 [41].
It is well known that the work functions of TiO 2 and SnO 2 are 5.58 eV and 4.9 eV, respectively.When TiO 2 comes into contact with SnO 2 , significant electron transfer occurs at their interface, with electrons moving from SnO 2 to TiO 2 .This charge transfer alters the electronic distribution at the interface, optimizing the surface electronic structure of the composite material.PL spectroscopy (Figure S6b) provides evidence of this electron transfer process.The interfacial charge transfer creates an electron depletion layer on the SnO 2 side and further bends the energy bands, resulting in a higher resistance for the sensing material compared to a pure SnO 2 sensor, consistent with our observations (Figure 6b).Due to the different electronic affinities and chemical properties of SnO 2 and TiO 2 , interfacial charge transfer can modulate the surface chemical state of the composite material.This enhances the selectivity of the composite material towards specific gas molecules, enabling accurate identification and detection of target gases in complex gaseous environments.
According to the literature, the adsorption of ethanol on the SnO 2 -TiO 2 surface actually involves both physical and chemical adsorption processes.Firstly, at temperatures below 100 • C, oxygen in the air is physically adsorbed onto the SnO 2 -TiO 2 surface, where it captures electrons to form chemically adsorbed O 2 − (ads), as described by Formula (3): At room temperature, the reaction rate of this process is very slow.However, as the temperature rises, the rate of chemical adsorption of oxygen increases, and O 2 − (ads) can be transformed into more stable O − (ads) and O 2− (ads) through Formulas ( 4) and (5) [42,43]  These chemically adsorbed oxygen ions create an electron depletion layer on the SnO 2 -TiO 2 surface, resulting in an increase in the resistance of SnO 2 -TiO 2 .When exposed to an ethanol atmosphere, ethanol molecules adsorb onto the SnO 2 -TiO 2 surface and undergo a redox reaction with the chemically adsorbed O − (ads), as described by Formula (6), Here, ethanol was oxidized to form CO 2 and H 2 O [29,44].This reaction releases electrons back into the conduction band of SnO 2 -TiO 2 , causing the potential barrier to decrease, the depletion layer to narrow, and the resistance to decrease.The chemical adsorption process of ethanol molecules on the SnO 2 -TiO 2 surface is illustrated in Figure 10.
Therefore, compared to single SnO 2 , TiO 2 , ST3-1, and ST1-3, ST1-1 exhibit stronger adsorption for oxygen and ethanol.In particular, the reactive oxygen on the surface facilitates the reaction with ethanol, resulting in changes in resistance and achieving efficient ethanol response.Additionally, the selective adsorption and oxidation of ethanol over SnO 2 and TiO 2 facilitate the selective detection of ethanol among different gases [7,8,24].

Figure 1 .
Figure 1.Schematic diagrams of (a) ball milling process; (b) MEMS gas sensitive chip preparation and test.

Figure 1 .
Figure 1.Schematic diagrams of (a) ball milling process; (b) MEMS gas sensitive chip preparation and test.

Figure 2 .
Figure 2. (a,b) are the SEM images of the original MEMS substrate, (c-f) are the SEM images of the gas sensitive film of ST1-1 sensor.
Figure3a,b present TEM images of SnO2 and TiO2, respectively.It can be observed that the particle sizes of SnO2 and TiO2 after ball milling are around tens of nanometers, but the particles are not uniform and tend to agglomerate.Figure3cdemonstrates that the particle size of the ball-milled ST1-1 composite material is mainly concentrated at about 30 nm, indicating a more uniform distribution.ST1-3 and ST3-1 also show similar results (FigureS6).Figure3ddisplays an HRTEM image of the ST1-1 composite material, showing a lattice spacing of 0.265 nm for SnO2 (101) and 0.169 nm for TiO2 (211), indicating a very close spatial contact between SnO2 and TiO2.EDS results confirm that the ST1-1 composite material consists of Ti, Sn, and O, as shown in Figure3.Figure3a-d further demonstrate that the interface consists of SnO2 nanoparticles and TiO2 nanoparticles.The energy spectrum of the ST1-1 composite material reveals a uniform distribution of Sn and Ti throughout the entire region (Figures3e-h and S7), indicating a relatively uniform spatial distribution of SnO2 and TiO2 nanoparticles after ball milling.

Figure 2 .
Figure 2. (a,b) are the SEM images of the original MEMS substrate, (c-f) are the SEM images of the gas sensitive film of ST1-1 sensor.
Figure3a,b present TEM images of SnO 2 and TiO 2 , respectively.It can be observed that the particle sizes of SnO 2 and TiO 2 after ball milling are around tens of nanometers, but the particles are not uniform and tend to agglomerate.Figure3cdemonstrates that the particle size of the ball-milled ST1-1 composite material is mainly concentrated at about 30 nm, indicating a more uniform distribution.ST1-3 and ST3-1 also show similar results (FigureS6).Figure3ddisplays an HRTEM image of the ST1-1 composite material, showing a lattice spacing of 0.265 nm for SnO 2 (101) and 0.169 nm for TiO 2 (211), indicating a very close spatial contact between SnO 2 and TiO 2 .EDS results confirm that the ST1-1 composite material consists of Ti, Sn, and O, as shown in Figure3.Figure3a-dfurther demonstrate that the interface consists of SnO 2 nanoparticles and TiO 2 nanoparticles.The energy spectrum of the ST1-1 composite material reveals a uniform distribution of Sn and Ti throughout the entire region (Figure 3e-h and Figure S7), indicating a relatively uniform spatial distribution of SnO 2 and TiO 2 nanoparticles after ball milling.The crystal structure and composition of the synthesized samples were analyzed by XRD, and the results are shown in Figure 4.The diffraction peaks located at 26.56 • , 33.82 • , 38.82 • , 51.50 • , and 65.42 • correspond to the (100), (101), (200), (211), and (301) crystal planes of SnO 2 (JCPDS #41-1445), respectively, indicating that the crystal form of SnO 2 is rutile [28].The diffraction peaks at 25.6 • , 38.1 • , 48.5 • , 55.6 • , and 62.9 • correspond to the (101), (004), (200), (211), and (204) crystal planes of TiO 2 (JCPDS #21-1276), respectively, indicating a rutile phase of TiO 2 [30].When SnO 2 and TiO 2 are mixed, the XRD spectrum mainly shows diffraction peaks of SnO 2 and TiO 2 , and no new diffraction peaks are formed, indicating that no chemical reaction has occurred to form a new phase.
further demonstrate that the interface consists of SnO2 nanoparticles and TiO2 nanoparticles.The energy spectrum of the ST1-1 composite material reveals a uniform distribution of Sn and Ti throughout the entire region (Figures3e-h and S7), indicating a relatively uniform spatial distribution of SnO2 and TiO2 nanoparticles after ball milling.

Figure 8 .
Figure 8. Selective testing of five gas sensors.

Figure 8 .
Figure 8. Selective testing of five gas sensors.The sensing performance of the ST1-1 sensor to ethanol at a working temperature of 288 • C was further investigated.Firstly, the response and recovery of the ST1-1 sensor to different concentrations (10-60 ppm) of ethanol were studied.The results, shown in Figure9a, indicate that the response value increases linearly with increasing gas concentration from 10 ppm to 60 ppm, with a fitted line slope of 0.34 (Figure9b).The response speed and recovery speed of the MEMS sensor can be evaluated by response time and recovery time.As shown in Figure9c, the response time of the ST1-1 sensor at 288 • C is approximately 11.8 s, and the recovery time is approximately 60.1 s, indicating relatively fast response and recovery speeds of the ST1-1.The stability of the ST1-1 sensor is demonstrated in Figure9d.There is no significant difference in the response values during six cycles of testing under 50 ppm ethanol conditions, indicating excellent signal reproducibility and short-term repeatability of the ST1-1 sensor.Figure9eshows the long-term stability of the ST1-1 sensor tested over 30 days under the same environmental conditions with 50 ppm ethanol.No significant drift or downward trend was observed within 30 days, indicating good long-term stability.Additionally, the effects of humidity on sensing performance and initial resistance were studied.The changes in the response and initial resistance of the ST1-1 sensor are shown in Figure9f, with an ambient temperature of 25 • C and a relative humidity (RH) range of 20-90%.Both parameters show small fluctuations in the low RH range of 25-55%, a significant decrease in the 55-75% range, and a slow decrease in the 75-90% range, requiring temperature drift correction.Consistency is also one of the important indicators of sensor performance.FigureS11shows that three MEMS gas sensors prepared under the same conditions respond almost identically to 50 ppm C 2 H 5 OH, illustrating that EHD technology provides a reliable and effective method for preparing gas sensors with excellent consistency.

Figure 9 .
Figure 9. Gas sensing performance of the ST1-1 sensor.(a) Response to 10 -60 ppm of ethanol and corresponding (b) linear fitting curve; (c) making their selectivity towards 50 ppm ethanol; (d) repeatability testing in 50 ppm of ethanol; (e) long-term stability in 30 days; and the (f) The effect of humidity on response and initial resistance in 50 ppm of ethanol.

Figure 9 .
Figure 9. Gas sensing performance of the ST1-1 sensor.(a) Response to 10 -60 ppm of ethanol and corresponding (b) linear fitting curve; (c) making their selectivity towards 50 ppm ethanol; (d) repeatability testing in 50 ppm of ethanol; (e) long-term stability in 30 days; and the (f) The effect of humidity on response and initial resistance in 50 ppm of ethanol.

Table 1 .
Central positions and relative percentages of OL and OV.

Table 1 .
Central positions and relative percentages of O L and O V .

Table 2 .
A comparison of sensing performance between current work and previously reported results over MOS-based MEMS sensor.

Table 2 .
A comparison of sensing performance between current work and previously reported results over MOS-based MEMS sensor.